MedGen

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high. [from GeneReviews]

Brugada syndrome is characterized by cardiac conduction abnormalities (ST segment abnormalities in leads V1-V3 on EKG and a high risk for ventricular arrhythmias) that can result in sudden death. Brugada syndrome presents primarily during adulthood, although age at diagnosis may range from infancy to late adulthood. The mean age of sudden death is approximately 40 years. Clinical presentations may also include sudden infant death syndrome (SIDS; death of a child during the first year of life without an identifiable cause) and sudden unexpected nocturnal death syndrome (SUNDS), a typical presentation in individuals from Southeast Asia. Other conduction defects can include first-degree AV block, intraventricular conduction delay, right bundle branch block, and sick sinus syndrome. [from GeneReviews]

Autosomal recessive pseudohypoaldosteronism type I, including PHA1B1, is characterized by renal salt wasting and high concentrations of sodium in sweat, stool, and saliva. The disorder involves multiple organ systems and is especially threatening in the neonatal period. Laboratory evaluation shows hyponatremia, hyperkalemia, and increased plasma renin activity with high serum aldosterone concentrations. Respiratory tract infections are common in affected children and may be mistaken for cystic fibrosis (CF; 219700). Aggressive salt replacement and control of hyperkalemia results in survival, and the disorder appears to become less severe with age (review by Scheinman et al., 1999). A milder, autosomal dominant form of type I pseudohypoaldosteronism (PHA1A; 177735) is caused by mutations in the mineralocorticoid receptor gene (MCR, NR3C2; 600983). Gitelman syndrome (263800), another example of primary renal tubular salt wasting, is due to mutation in the thiazide-sensitive sodium-chloride cotransporter (SLC12A3; 600968). Hanukoglu and Hanukoglu (2016) provided a detailed review of the ENaC gene family, including structure, function, tissue distribution, and associated inherited diseases. [from OMIM]

Hypokalemic periodic paralysis (hypoPP) is a condition in which affected individuals may experience paralytic episodes with concomitant hypokalemia (serum potassium <3.5 mmol/L). The paralytic attacks are characterized by decreased muscle tone (flaccidity) more marked proximally than distally with normal to decreased deep tendon reflexes. The episodes develop over minutes to hours and last several minutes to several days with spontaneous recovery. Some individuals have only one episode in a lifetime; more commonly, crises occur repeatedly: daily, weekly, monthly, or less often. The major triggering factors are cessation of effort following strenuous exercise and carbohydrate-rich evening meals. Additional triggers can include cold, stress/excitement/fear, salt intake, prolonged immobility, use of glucosteroids or alcohol, and anesthetic procedures. The age of onset of the first attack ranges from two to 30 years; the duration of paralytic episodes ranges from one to 72 hours with an average of nearly 24 hours. Long-lasting interictal muscle weakness may occur in some affected individuals and in some stages of the disease and in myopathic muscle changes. A myopathy may occur independent of paralytic symptoms and may be the sole manifestation of hypoPP. [from GeneReviews]

Bronchiectasis with or without elevated sweat chloride-1 (BESC1) is characterized by dilation of the airways arising from chronic bronchial inflammation accompanied by chronic cough, purulent sputum, and recurrent respiratory tract infections. Severity is variable, and some patients may be identified in adulthood and have normal respiratory function (Sheridan et al., 2005, Fajac et al., 2008). Genetic Heterogeneity of Bronchiectasis with or without Elevated Sweat Chloride Bronchiectasis with or without elevated sweat chloride-2 (BESC2; 613021) is caused by mutation in the gene encoding the alpha subunit of the epithelial sodium channel (SCNN1A; 600228) on chromosome 12p13, and BESC3 (613071) is caused by mutation in the gene encoding the gamma subunit (SCNN1G; 600761) on chromosome 16p12. Bronchiectasis and elevated sweat chloride associated with pancreatic exocrine dysfunction and infertility are also features of cystic fibrosis (CF; 219700), which is caused by mutation in the CFTR gene (602421). [from OMIM]

Progressive familial heart block type I (PFHBI, PFHB1) is an autosomal dominant cardiac bundle branch disorder that may progress to complete heart block (Brink and Torrington, 1977; van der Merwe et al., 1986; van der Merwe et al., 1988). It is defined on electrocardiogram by evidence of bundle branch disease, i.e., right bundle branch block, left anterior or posterior hemiblock, or complete heart block, with broad QRS complexes. Progression has been shown from a normal electrocardiogram to right bundle branch block and from the latter to complete heart block. These electrocardiographic features differentiate PFHB type I from progressive familial heart block type II (PFHBII, PFHB2; 140400), in which the onset of complete heart block is associated with narrow complexes. Electrocardiographically the changes represent, respectively, bundle branch disease (PFHB1) and atrioventricular nodal disease with an atrioventricular block and an idionodal escape rhythm (PFHB2). PFHBI is manifested symptomatically when complete heart block supervenes, either with dyspnea, syncopal episodes, or sudden death. Treatment, which is best managed by regular electrocardiographic follow-up, is by the timely implantation of a pacemaker (Brink et al., 1995). Genetic Heterogeneity of Progressive Familial Heart Block Type I Progressive familial heart block type IB (PFHB1B; 604559) is caused by mutation in the TRPM4 gene (606936) on chromosome 19q13.32. [from OMIM]

Sudden infant death syndrome (SIDS) is a diagnosis of exclusion which should be made only after a thorough autopsy without identification of a specific cause of death (Mage and Donner, 2004). Weese-Mayer et al. (2007) provided a detailed review of genetic factors that have been implicated in SIDS. The authors concluded that SIDS represents more than 1 entity and has a heterogeneous etiology most likely involving several different genetically controlled metabolic pathways. [from OMIM]

Generalized epilepsy with febrile seizures plus type 1 (GEFSP1) is an autosomal dominant neurologic disorder characterized by onset of seizures associated with fever in infancy or early childhood. There is wide phenotypic variability, even within families. In contrast to classic febrile seizures (see, e.g., FEB1, 121210), which affect approximately 3% of children under 6 years of age and typically spontaneously remit by age 6 years, patients with GEFSP1 either have febrile seizures extending beyond age 6 years or develop epilepsy with afebrile seizures. Other seizure types include absence seizures, partial seizures, myoclonic seizures, and atonic seizures. Some patients may have developmental delay after the onset of seizures (summary by Wallace et al., 1998 and Singh et al., 1999). Deprez et al. (2009) reviewed the genetics of epilepsy syndromes starting in the first year of life, and included a diagnostic algorithm. Genetic Heterogeneity of GEFS+ GEFS+ is a genetically heterogeneous disorder. See also GEFS+2 (604403), caused by mutation in the SCN1A gene (182389) on chromosome 2q24; GEFS+3 (see 607681), caused by mutation in the GABRG2 gene (137164) on chromosome 5q34; GEFS+5 (613060), associated with variation in the GABRD (137163) gene on chromosome 1p36; GEFS+9 (616172), caused by mutation in the STX1B gene (601485) on chromosome 16p11; GEFS+10 (618482), caused by mutation in the HCN1 gene (602780) on chromosome 5p12; GEFS+11 (602477), caused by mutation in the HCN2 gene (602781) on chromosome 19p13; and GEFS+12 (620755), caused by mutation in the SLC32A1 gene (616440) on chromosome 20q11. Several putative loci have also been identified; see GEFS+4 (609800), mapped to chromosome 2p24; GEFS+6 (612279), mapped to chromosome 8p23-p21; GEFS+7 (613863), mapped to chromosome 2q24; and GEFS+8 (613828), mapped to chromosome 6q16.3-q22.31. [from OMIM]

Ventricular fibrillation (VF) is said to cause more than 300,000 sudden deaths each year in the US alone. In approximately 5 to 12% of cases, there are no demonstrable cardiac or noncardiac causes to account for the episode, which is therefore classified as idiopathic ventricular fibrillation (IVF). Patients with a distinct form of VF called Brugada syndrome (see 601144) present with a characteristic electrocardiographic pattern, with right bundle branch block (RBBB) and elevation of ST segment in leads V1 to V3 and may account for 40 to 60% of all IVF cases (review by Chen et al., 1998). Mutations in the SCN5A gene were identified in patients with Brugada syndrome-1 (601144). Genetic Heterogeneity of Paroxysmal Familial Ventricular Fibrillation Another familial form of VF (VF2; 612956) is caused by mutation in the DPP6 gene (126141) on chromosome 7q26. [from OMIM]

The term 'sick sinus syndrome' encompasses a variety of conditions caused by sinus node dysfunction. The most common clinical manifestations are syncope, presyncope, dizziness, and fatigue. Electrocardiogram typically shows sinus bradycardia, sinus arrest, and/or sinoatrial block. Episodes of atrial tachycardias coexisting with sinus bradycardia ('tachycardia-bradycardia syndrome') are also common in this disorder. SSS occurs most often in the elderly associated with underlying heart disease or previous cardiac surgery, but can also occur in the fetus, infant, or child without heart disease or other contributing factors, in which case it is considered to be a congenital disorder (Benson et al., 2003). Genetic Heterogeneity of Sick Sinus Syndrome Sick sinus syndrome-2 (SSS2; 163800) is caused by mutation in the HCN4 gene (605206). Susceptibility to sick sinus syndrome-3 (SSS3; 614090) is influenced by variation in the MYH6 gene (160710). Sick sinus syndrome-4 (SSS4; 619464) is caused by mutation in the GNB2 gene (139390). [from OMIM]

Liddle syndrome is an autosomal dominant disorder characterized by early-onset salt-sensitive hypertension, hypokalemia, metabolic alkalosis, and suppression of plasma renin activity and aldosterone secretion (summary by Yang et al., 2014). Genetic Heterogeneity of Liddle Syndrome Liddle syndrome-2 (618114) is caused by mutation in the SCNN1G gene (600761), which encodes the ENaC gamma subunit. Liddle syndrome-3 (618126) is caused by mutation in the SCNN1A gene (600228), which encodes the ENaC alpha subunit. Hanukoglu and Hanukoglu (2016) provided a detailed review of the ENaC gene family, including structure, function, tissue distribution, and associated inherited diseases. [from OMIM]

Gitelman syndrome (GTLMNS) is an autosomal recessive renal tubular salt-wasting disorder characterized by hypokalemic metabolic alkalosis with hypomagnesemia and hypocalciuria. It is the most common renal tubular disorder among Caucasians (prevalence of 1 in 40,000). Most patients have onset of symptoms as adults, but some present in childhood. Clinical features include transient periods of muscle weakness and tetany, abdominal pains, and chondrocalcinosis (summary by Glaudemans et al., 2012). Gitelman syndrome is sometimes referred to as a mild variant of classic Bartter syndrome (607364). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364. [from OMIM]

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364. [from OMIM]

Fucosidosis is an autosomal recessive lysosomal storage disease caused by defective alpha-L-fucosidase with accumulation of fucose in the tissues. Clinical features include angiokeratoma, progressive psychomotor retardation, neurologic signs, coarse facial features, and dysostosis multiplex. Fucosidosis has been classified into 2 major types. Type 1 is characterized by rapid psychomotor regression and severe neurologic deterioration beginning at about 6 months of age, elevated sweat sodium chloride, and death within the first decade of life. Type 2 is characterized by milder psychomotor retardation and neurologic signs, the development of angiokeratoma corporis diffusum, normal sweat salinity, and longer survival (Kousseff et al., 1976). [from OMIM]

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal (or neonatal) forms of Bartter syndrome (e.g., BARTS1, 601678) typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). Genetic Heterogeneity of Bartter Syndrome Antenatal Bartter syndrome type 1 (601678) is caused by loss-of-function mutations in the butmetanide-sensitive Na-K-2Cl cotransporter NKCC2 (SLC12A1; 600839). Antenatal Bartter syndrome type 2 (241200) is caused by loss-of-function mutations in the ATP-sensitive potassium channel ROMK (KCNJ1; 600359). One form of neonatal Bartter syndrome with sensorineural deafness, Bartter syndrome type 4A (602522), is caused by mutation in the BSND gene (606412). Another form of neonatal Bartter syndrome with sensorineural deafness, Bartter syndrome type 4B (613090), is caused by simultaneous mutation in both the CLCNKA (602024) and CLCNKB (602023) genes. Also see autosomal dominant hypocalcemia-1 with Bartter syndrome (601198), which is sometimes referred to as Bartter syndrome type 5 (Fremont and Chan, 2012), caused by mutation in the CASR gene (601199). See Gitelman syndrome (GTLMN; 263800), which is often referred to as a mild variant of Bartter syndrome, caused by mutation in the thiazide-sensitive sodium-chloride cotransporter SLC12A3 (600968). [from OMIM]

Approximately 10% of patients with congenital hypothyroidism harbor inborn errors of metabolism in one of the steps for thyroid hormone synthesis in thyrocytes (Vono-Toniolo et al., 2005). Dyshormonogenesis can be caused by recessive defects at any of the steps required for normal thyroid hormone synthesis. In untreated patients thyroid dyshormonogenesis is typically associated with goitrous enlargement of the thyroid secondary to long-term thyrotropin (TSH; see 188540) stimulation. Park and Chatterjee (2005) reviewed the genetics of primary congenital hypothyroidism, summarizing the different phenotypes associated with known genetic defects and proposing an algorithm for investigating the genetic basis of the disorder. Genetic Heterogeneity of Thyroid Dyshormonogenesis Other forms of thyroid hormone dysgenesis include TDH2A (274500), caused by mutation in the thyroid peroxidase gene (TPO; 606765) on 2p25; Pendred syndrome, a form of thyroid hormone dysgenesis associated with deafness (TDH2B; 274600) and caused by mutation in the SLC26A4 gene (605646) on 7q31; TDH3 (274700), caused by mutation in the thyroglobulin gene (TG; 188450) on 8q24; TDH4 (274800), caused by mutation in the iodotyrosine deiodinase gene (IYD; 612025) on 6q25; TDH5 (274900), caused by mutation in the DUOXA2 gene (612772) on 15q21; and TDH6 (607200), caused by mutation in the DUOX2 gene (606759) on 15q21. [from OMIM]

Malignant hyperthermia susceptibility (MHS) is a pharmacogenetic disorder of skeletal muscle calcium regulation associated with uncontrolled skeletal muscle hypermetabolism. Manifestations of malignant hyperthermia (MH) are precipitated by certain volatile anesthetics (i.e., halothane, isoflurane, sevoflurane, desflurane, enflurane), either alone or in conjunction with a depolarizing muscle relaxant (specifically, succinylcholine). The triggering substances cause uncontrolled release of calcium from the sarcoplasmic reticulum and may promote entry of extracellular calcium into the myoplasm, causing contracture of skeletal muscles, glycogenolysis, and increased cellular metabolism, resulting in production of heat and excess lactate. Affected individuals experience acidosis, hypercapnia, tachycardia, hyperthermia, muscle rigidity, compartment syndrome, rhabdomyolysis with subsequent increase in serum creatine kinase (CK) concentration, hyperkalemia with a risk for cardiac arrhythmia or even cardiac arrest, and myoglobinuria with a risk for renal failure. In nearly all cases, the first manifestations of MH (tachycardia and tachypnea) occur in the operating room; however, MH may also occur in the early postoperative period. There is mounting evidence that some individuals with MHS will also develop MH with exercise and/or on exposure to hot environments. Without proper and prompt treatment with dantrolene sodium, mortality is extremely high. [from GeneReviews]

Developmental and epileptic encephalopathy-52 (DEE52) is a severe autosomal recessive seizure disorder characterized by infantile onset of refractory seizures with resultant delayed global neurologic development. Affected individuals have impaired intellectual development and may have other persistent neurologic abnormalities, including axial hypotonia and spasticity; death in childhood may occur (summary by Patino et al., 2009 and Ramadan et al., 2017). Some patients with DEE52 may have a clinical diagnosis of Dravet syndrome (607208), which is characterized by the onset of seizures in the first year or 2 of life after normal early development. Developmental delay, impaired intellectual development, and behavioral abnormalities usually become apparent later between 1 and 4 years of age. Dravet syndrome may also include 'severe myoclonic epilepsy in infancy' (SMEI) (summary by Patino et al., 2009). For a discussion of genetic heterogeneity of DEE, see 308350. [from OMIM]